It really depends what you plan on doing with it.
If you want to do a high volume of printing, that lasts longer, I would suggest a screen printing set up. Heat press transfers don't usually hold up to washing and such as often.
However, if you want to do even shirts, then a heat press machine might be the way to go. With one of these you can bring unprinted shirts and the pre-printed transfers to the event and simply press them as needed. That way you won't have shirts that are of no use to you later. You can just put the blank shirts back into your stock.
Classic Screen Print
8176 Center St Ste C
La Mesa CA 91942
They do screen print for Padres, Chargers, Aztecs etc